1. Field of the Invention
The present invention relates to a sputtering target used for the deposition of an oxide recording layer constituting an optical recording medium and to a manufacturing method for the same. The present invention also relates to an optical recording medium manufactured by using the sputtering target, particularly a recordable optical recording medium capable of high-density recording even in the blue laser's wavelength range and to a manufacturing method for the same.
2. Description of the Related Art
Blue lasers that enable ultrahigh-density recording have been rapidly developed for the provision of recordable optical recording media capable of recording and reproducing at wavelengths equal to or shorter than those of blue lasers. Correspondingly, recordable optical recording media supporting such blue lasers have also been developed.
The inventors of this application discloses in Japanese Patent Application Laid-Open (JP-A) No. 2005-108396 that a recordable optical recording medium using a film containing Bi, Fe, and O shows excellent characteristics, and that the sputtering method is used for the formation of this film. The present inventors also disclose a sputtering target containing Bi, Fe, and O in JP-A No. 2006-097126.
The sputtering method has been widely known as one of the vapor growth methods for depositing a thin film, and has been applied to industrial manufacture of thin films. In the sputtering method a target material containing the same ingredients as a film to be deposited is prepared, and argon ions generated via grow discharge are bombarded into the target material to kick out its constituent ions, allowing them to be deposited onto a substrate to form a film. Vapor deposition methods are not preferable because oxides generally have high melting points; therefore, radio-frequency sputtering is often used that involves application of high-frequency waves.
As a known technology, for example, JP-A No. 11-92922 discloses a Bi oxide-based sputtering target used for the deposition of a dielectric film, but this patent literature fails to describe an Fe-containing sputtering target. For this reason, the disclosure of the patent literature provides no basis for accomplishing the present invention.
In addition, Japanese Patent Application Publication (JP-B) No. 02-42899 discloses s sputtering target used to form a thin film made of Bi3Fe5O12, but the invention disclosed in the patent literature differs from the present invention in that the former intends to produce a film with a so-called garnet structure that imparts high magnetooptical effects. Moreover, this patent literature fails to describe a relationship between packing density and media characteristics.
Japanese Patent (JP-B) No. 2769153 discloses a sputtering target for thin films for recording media, which the sputtering target consists at least of Te, Ge and Sb and has a packing density of 80% or more. This literature also describes the fact that, if packing density of 80% or more is achieved, there is no change in the composition after sputtering the target for 1 hour, i.e., it is possible to obtain the same composition for a long period of time after 1-hour sputtering, while presenting comparison data for 70%, 80%, and 90% packing densities. The composition of this sputtering target, however, is different from that of the sputtering target of the present invention and provides no detailed comparison data for packing density of greater than 90%. Thus, the patent literature provides no basis for establishing that there is a threshold in packing density at which media characteristics change—the present inventors have established that such a threshold exists at 96%, a percentage almost close to 100%.
A problem associated with oxide targets used in the sputtering method is that their film deposition rates are low compared to other species such as metals and the like, and that they are prone to fracture.
To solve this problem, the present inventors demonstrated in JP-A No. 2006-097126 that reduced film deposition time, high target strength, and desired film composition can be realized by setting packing density to within the range of 65% to 96%.
Their extensive studies have revealed that continuous manufacture of optical recording media using the oxide target disclosed in JP-A No. 2006-097126 poses a problem regarding the reproducibility of media characteristics. For example, an optimal recording power differs between initially manufactured optical recording media and the 1000th one.
Moreover, since it is difficult to adopt direct-current sputtering for high-electrical resistance materials (e.g., oxides) for film deposition, radio-frequency sputtering is used.
Radio-frequency sputtering requires an expensive device for controlling high-frequency power, and radio-frequency sputtering devices are often big. On the other hand, direct-current sputtering can achieve film deposition with an inexpensive device. Moreover, radio-frequency sputtering offers low film deposition rates and low efficiency compared to direct-current sputtering. Accordingly, direct-current sputtering is desirable for the sputtering of Bi oxide-based materials, but at present, their inherent high-electrical resistance properties make the employment of direct-current sputtering difficult.
Conventionally, there have been made two attempts to replace radio-frequency sputtering by direct-current sputtering: (1) an attempt to make the target material electrically conductive to make direct-current sputtering applicable; and (2) an attempt to optimize the direct current waveforms. Furthermore, the combination of these attempts (1) and (2) has been employed.
There are several known strategies as to the method of reducing the electrical resistance of a target material by making it electrically conductive: (1) a strategy where a main material (Si) is doped with a substance (typified by Cr) with an electrical resistance lower than that of the main material (Si) (see JP-A No. 02-265052); (2) a strategy where a main material (Si) is heavily doped with a substance (PB) that reduces electrical resistance (see JP-A No. 05-36142); a strategy where a portion (oxygen ingredient) is removed from the main material (PLZT-PZT-based material) to reduce its electrical resistance (see JP-A No. 06-330297), etc.
With respect to the foregoing attempt (2), an attempt has been made to prevent such abnormal discharge as arching by means of a method of intermittently applying direct-current voltage rather than continuously (see JP-B No. 2578815) and/or a method of intermittently applying negative voltage (see JP-A No. 08-67980). This strategy is carried out by using the method of reducing the electrical resistance of target material together or by applying reverse-bias voltage (see JP-A No. 07-90573), because electric charges at the surface of the target material need to cancel when the target material is intended to remain insulating.
However, even when the a sputtering target has been made initially conductive by reducing its electrical resistance through the foregoing strategy (1), there would be a problem that because of the presence of reactive gases such as oxygen gas and nitrogen gas used upon film deposition, the conductivity of the target surface changes as the reaction proceeds, or a problem that when atmospheric air has been introduced in the vacuum chamber, the sputtering target reacts with oxygen in the air to cause conductivity change at the target surface.
To solve this problem JP-A No. 07-180045 discloses as a conductive target for direct-current magnetron sputtering, a sputtering target that contains at least 35% by volume of yttria based on the total amount of amorphous carbon and sputtering target. Amorphous carbon can be obtained by firing an organic complex to thermally decompose its ingredients other than carbon. After that, carbon films are formed in the pores created after the ingredients other than carbon were removed, thereby making the sputtering target conductive. In addition, the patent literature describes that the density of the sputtering target is about 45-70% of the theoretical value.
When carbon is used for this conductive sputtering target as a substance to make the target conductive, it seems that change in the conductivity of the target surface is small because there are no carbon oxides or carbon nitrides present as solid material. In the invention of this patent literature, however, there is a concern that abnormal discharge may occur due to pores created after removal of ingredients other than carbon. Moreover, since creation of such pores cannot be prevented, the density and strength of the sputtering target cannot be increased. Thus, for example, there is a concern that the sintered body may crack upon bonding to a backing plate due to reduced mechanical strength, and that high-rate film deposition cannot be realized because high-output sputtering causes cracks in the sputtering target.
The strategy (2) described above, where sputtering involves intermittent application of direct-current voltage and/or intermittent application of reverse-bias voltage at predetermined intervals, cannot avoid the use of a complicated device, which makes the device expensive. Thus there is no advantage in the adoption of direct-current sputtering.
JP-A No. 2000-264731 describes another attempt to achieve low electrical resistance by adding either diamond structure-free carbon or a conductive inorganic compound other than metal.
Although it is obvious that deposition of a film containing Bi, Fe and O can be done by direct-current sputtering using any of the conventional methods described above, it still remains unknown in this invention whether the resultant film can maintain characteristics as a recording layer.
In addition, JP-A No. 11-92922 discloses as a sputtering target for the deposition of dielectric films, a Bi oxide-based target, but this patent literature does not describe any Fe-containing target. If the constituent elements of a film to be deposited change, the composition of the sputtering target for that film needs to be changed correspondingly. This is because the composition or constituent compound of the sputtering target differs from that of the film to be deposited. The finding disclosed in the patent literature provides no basis for the present invention.
JP-B No. 02-42899 discloses a Bi3Fe5O12 sputtering target for thin film deposition, but this sputtering target in this patent literature is one intended for ion beam sputtering for forming a thin film with a so-called garnet structure that imparts high magnetooptical effects, and no sputtering targets that make direct-current sputtering applicable are described.
To realize a recordable optical recording medium capable of high-density recording, it is necessary for that medium to include a film that contains Bi, Fe and O and has a stable structure and composition. To produce such a film a proper sputtering target needs to be prepared. The form and structure of the compound constituting the sputtering target, impurities contained therein, etc., affect the composition, crystallization properties, etc., of a film to be deposited. For this reason, it is required to select such a target-constituting compound that is suitable for the characteristics of a film required. Moreover, film deposition needs to be carried out by direct-current sputtering because recordable optical recording media need to be fabricated inexpensively.
However, it still remains unknown in this invention whether the strength of the target and the film deposition rate can be increased.
For increased film deposition rates, magnetron sputtering is often used together, in which a magnetic field generation means is placed under the sputtering target.
Magnetron sputtering increases the film deposition rate in the following way: Ar plasma is converged near the sputtering target by means of a magnetic field generated from the magnetic field generation means placed under the target, whereby the number of ions bombarding into the target surface increases, so too does the number of atoms to be sputtered. Both radio-frequency magnetron sputtering and direct-current magnetron sputtering can be used.
In magnetron sputtering, it is required that magnetic fluxes generated from the magnetic field generation means leak to the target surface side. Accordingly, if magnetic permeability of the sputtering target is high, it results in an increased number of magnetic lines passing through the target, and therefore, film deposition rates decrease due to insufficient magnetic field intensity of the leaked the magnetic flux. Thus, for a greater level of magnetic flux leakage, a sputtering target with a high magnetic permeability needs to be made thin and/or has a structure that facilitates magnetic flux leakage so that leaked magnetic fluxes are secured.
In addition, if sputtering targets are made thin, the sputtering time that can be spent for one piece decreases, resulting in a high frequency of replacement of sputtering target which entails opening and evacuation of the sputtering chamber. For this reason, manufacturing costs will increase due to poor productivity.
Furthermore, when sputtering targets are to be prepared by means of sintering method, the resulting sputtering targets need to be grinded for the removal of an oxidized region near the surface before they are put on the market. The degree of grinding is about 1 mm from the surface regardless of their thickness; therefore, the thinner the sputtering target, the more the proportion of the portion discarded without being sputtered, which also increases the manufacturing costs.
As another known technology, JP-B No. 3098204 and the like disclose a sputtering target where its magnetic permeability is successfully reduced by formulating a compound comprising Fe and other elements.
JP-A No. 06-184740 and the like disclose a sputtering target made of alloy particles made from a compound comprising an Fe group element and other elements, the reduced magnetic permeability of the sputtering being achieved by setting the average particle diameter of the alloy particles to within 200 μm or less.
Although it is obvious that application of these known technologies to a sputtering target containing Bi, Fe and O enables employment of magnetron sputtering, it still remains unknown whether an optical recording medium offering excellent characteristics can be fabricated using a film deposited in that way. Thus, these conventional technologies provide no basis for the present invention.
If the constituent elements of a film to be deposited change, the composition of the sputtering target for that film needs to be changed correspondingly. This is because the composition or constituent compound of the sputtering target differs from that of the film to be deposited. The findings disclosed in the patent literatures above provide no basis for the present invention.
JP-B No. 02-42899 sets forth description regarding a Bi3Fe5O12 sputtering target for thin film deposition, where Bi2O3 and α-Fe2O3 are mixed together and sintered for the preparation of the target. The invention disclosed in this patent literature, however, intends to produce a film with a so-called garnet structure that imparts high magnetooptical effects, failing to describe magnetron sputtering, and thus differs from the present invention.
To realize a recordable optical recording medium capable of high-density recording, it is necessary to produce a film that contains Bi, Fe and O (oxygen) and has a stable structure and composition. In view of productivity and manufacturing costs, film deposition rates and availability of sputtering target need to be taken into consideration. To achieve this, it is required to prepare a proper film by using a proper sputtering target. The form and structure of the compound constituting the sputtering target, impurities contained therein, etc., affect the composition, crystallization properties, etc., of a film to be deposited. For this reason, it is required to select such a target-constituting compound that is suitable for the characteristics of a film required. Moreover, film deposition needs to be carried out using high-film deposition rate methods because recordable optical recording media need to be manufactured inexpensively.